I think this might be the perfect kids backpack! Its the On The Go Backpack by Heather Bailey, which I bought after seeing it in Heather's booth at Quilt Market last May.
Eva chose the fabrics (this is where having a large stash is very useful) and I whipped it up while she was at school one day. The instructions were very clear it went together perfectly. She loves it, it was used immediately, and think a couple the other girls might want one. If you want to make a backpack I highly recommend this pattern!